Vapor compression systems circulate refrigerant in a closed loop system to transfer heat from one external medium to another external medium. Vapor compression systems are used in air-conditioning, heat pump, and refrigeration systems. FIG. 1 depicts a conventional vapor compression heat transfer system 100 that operates though the compression and expansion of a refrigerant fluid. The vapor compression system 100 transfers heat from a first external medium 150, through a closed-loop, to a second external medium 160. Fluids include liquid and/or gas phases.
A compressor 110 or other compression device reduces the volume of the refrigerant, thus creating a pressure difference that circulates the refrigerant through the loop. The compressor 110 may reduce the volume of the refrigerant mechanically or thermally. The compressed refrigerant is then passed through a condenser 120 or heat exchanger, which increases the surface area between the refrigerant and the second external medium 160. As heat transfers to the second external medium 160 from the refrigerant, the refrigerant contracts in volume.
When heat transfers to the compressed refrigerant from the first external medium 150, the compressed refrigerant expands in volume. This expansion is often facilitated with a metering device 130 including an expansion device and a heat exchanger or evaporator 140. The evaporator 140 increases the surface area between the refrigerant and the first external medium 150, thus increasing the heat transfer between the refrigerant and the first external medium 150. The transfer of heat into the refrigerant causes at least a portion of the expanded refrigerant to undergo a phase change from liquid to gas. The heated refrigerant is then passed back to the compressor 110 and the condenser 120, where at least a portion of the heated refrigerant undergoes a phase change from gas to liquid when heat transfers to the second external medium 160.
The closed-loop heat transfer system 100 may include other components, such as a compressor discharge line 115 joining the compressor 110 and the condenser 120. The outlet of the condenser 120 may be coupled to a condenser discharge line 125, and may connect to receivers for the storage of fluctuating levels of liquid, filters and/or desiccants for the removal of contaminants, and the like (not shown). The condenser discharge line 125 may circulate the refrigerant to one or more metering devices 130.
The metering device 130 may include one or more expansion devices. An expansion device may be any device capable of expanding, or metering a pressure drop in the refrigerant at a rate compatible with the desired operation of the system 100. Useful expansion devices include thermal expansion valves, capillary tubes, fixed and adjustable nozzles, fixed and adjustable orifices, electronic expansion valves, automatic expansion valves, manual expansion valves, and the like. The expanded refrigerant enters the evaporator 140 in a substantially liquid state with a small vapor fraction.
The refrigerant exiting the expansion portion of the metering device 130 passes through an expanded refrigerant transfer system 135, which may include one or more refrigerant directors 136, before passing to the evaporator 140. The expanded refrigerant transfer system 135 may be incorporated with the metering device 130, such as when the metering device 130 is located close to or integrated with the evaporator 140. Thus, the expansion portion of the metering device 130 may be connected to one or more evaporators by the expanded refrigerant transfer system 135, which may be a single tube or include multiple components. The metering device 130 and the expanded refrigerant transfer system 135 may have fewer or additional components, such as described in U.S. Pat. Nos. 6,751,970 and 6,857,281, for example.
One or more refrigerant directors may be incorporated with the metering device 130, the expanded refrigerant transfer system 135, and/or the evaporator 140. Thus, the functions of the metering device 130 may be split between one or more expansion device and one or more refrigerant directors and may be present separate from or integrated with the expanded refrigerant transfer system 135 and/or the evaporator 140. Useful refrigerant directors include tubes, nozzles, fixed and adjustable orifices, distributors, a series of distributor tubes, valves, and the like.
The evaporator 140 receives the expanded refrigerant and provides for the transfer of heat to the expanded refrigerant from the first external medium 150 residing outside of the closed-loop heat transfer system 100. Thus, the evaporator or heat exchanger 140 facilitates in the movement of heat from one source, such as ambient temperature air, to a second source, such as the expanded refrigerant. Suitable heat exchangers may take many forms, including copper tubing, plate and frame, shell and tube, cold wall, and the like. Conventional systems are designed, at least theoretically, to completely convert the liquid portion of the refrigerant to vaporized refrigerant from heat transfer within the evaporator 140. In addition to the heat transfer converting liquid refrigerant to a vapor phase, the vaporized refrigerant may become superheated, thus having a temperature in excess of the boiling temperature and/or increasing the pressure of the refrigerant. The refrigerant exits the evaporator 140 through an evaporator discharge line 145 and returns to the compressor 110.
In conventional vapor compression systems, the expanded refrigerant enters the evaporator 140 at a temperature that is significantly colder than the temperature of the air surrounding the evaporator. As heat transfers to the refrigerant from the evaporator 140, the refrigerant temperature increases in the later or downstream portion of the evaporator 140 to a temperature above that of the air surrounding the evaporator 140. This rather significant temperature difference between the initial or inlet portion of the evaporator 140 and the later or outlet portion of the evaporator 140 may lead to oiling and frosting problems at the inlet portion.
A significant temperature gradient between the inlet portion of the evaporator 140 and the outlet portion of the evaporator 140 may lead to lubricating oil, which is intended to be carried by the refrigerant, separating from the refrigerant, and “puddling” in the inlet portion of the evaporator. Oil-coated portions of the evaporator 140 substantially reduce the heat transfer capacity and result in reduced heat transfer efficiency.
If the expanded refrigerant entering the evaporator 140 cools the initial portion of the evaporator 140 to below 0° C., frost may form if there is moisture in the surrounding air. To obtain maximum evaporator performance from these systems, the spacing between the fins of the evaporator 140 is narrow. However, any frost that forms on these narrow fins quickly blocks airflow through the evaporator 140, thus, reducing heat transfer to the second external medium 160 and rapidly reducing operating efficiency. Conventional heat transfer systems may be designed where the temperature of the evaporator should never drop below 0° C. In systems of this type, the average temperature of the evaporator 140 during operation of the compressor 110 ranges from about 4° to about 8° C., so that the refrigerant in the initial portion of the evaporator 140 is maintained above 0° C. However, if conditions change, such as a drop in the temperature of the air surrounding the evaporator 140, the initial portion of the evaporator 140 may drop below 0° C. and frost.
To guard against such frosting, these systems may be equipped to shutdown if the air surrounding the evaporator 140 drops below a specific temperature. Thus, the system may passively defrost by turning off the compressor 110 so that heat transfers from the first external medium 150 into the evaporator 140. Lacking the ability to actively remove the frost from the evaporator 140 through the transfer of heat from an external source, such as with an electric heating element, or by passing previously heated refrigerant, such as taken from the high pressure side of the system, through the evaporator 140 during operation, the system 100 typically shuts down to prevent failure. Active defrosting does not include time periods when the compressor 110 is not operating, unless heat is being supplied to the evaporator 140 by a source other than the refrigerant, compressor 110, or condenser 120 when the compressor 110 is not operating.
Although air conditioning system evaporators typically operate at temperatures higher than 0° C., the temperature of an air conditioning evaporator may drop below 0° C. if the temperature of the air passing through the evaporator decreases. Furthermore, as the temperature required for food storage has decreased from about 7.2° C. to 5° C., the need to operate evaporators at 0° C. and lower has increased. However, when conventional air conditioning evaporator temperatures unexpectedly drop to 0° C. or below or when conventional heat transfer systems are equipped with evaporators intended to operate at or below 0° C. for refrigeration, the conventional systems generally have expanded refrigerant in the initial portion of the evaporator 140 at a temperature below the dew point of the ambient air, resulting in moisture condensation and freezing on the evaporator during operation. As this frost encloses a portion of the evaporator's surface, thus isolating the frosted surface from direct contact with the ambient air. Consequently, airflow over and/or through the evaporator 140 is reduced and cooling efficiency decreases. As the frost built up during on-cycles of the compressor 110 may not substantially melt during off-cycles of the compressor 110, defrost cycles are used to remove the frost and restore efficiency to the system 100 when operated at or below 0° C.
Conventional heat transfer systems may passively defrost by turning off the compressor 110 or may actively defrost by applying heat to the evaporator 140 during defrost cycles. As the compressor 110 is off during passive defrosting, the rate at which the system 100 can cool is reduced. For active defrosting, the required heat may be provided to the evaporator 140 by any means compatible with the operation of the system 100, including electric heating elements, heated gasses, heated liquids, infrared irradiation, and the like. Both passive and active defrosting systems require a larger vapor compression system than would be required if the system did not have to suspend cooling to defrost. Furthermore, active methods require energy to introduce heat to the evaporator 140, and additional energy to remove the introduced heat with the compressor 110 and the condenser 120 during the next cooling cycle. Thus, active defrosting reduces the overall efficiency of the system 100 because it must heat to defrost and then re-cool to operate.
In addition to the disadvantages of increased size and reduced cooling rate or efficiency attributable to the defrost requirements of conventional heat transfer systems, conventional systems also lose efficiency due to the lower levels of relative humidity achieved during operation. As moisture forms on a surface that is colder than the dew point of the surrounding air, frost will build up on a surface that is consistently colder than the dew point of the surrounding air and below 0° C. if the velocity of the air is sufficiently low. Thus, conventional heat transfer systems consume energy to remove moisture from the surrounding air and to lower the dew point of the air surrounding the evaporator. Cooling efficiency is reduced as energy consumed condensing moisture from the air is not spent cooling the air. As with the energy consumed to actively defrost and then re-cool the evaporator 140 for cooling duty, energy consumed removing water from the air is wasted. Additionally, active defrost cycles warm the cooled air at the evaporator, and with warming, the relative humidity of the air drops.
In addition to the energy consumed, a disadvantage of dehumidification is that any moisture-containing product present in the dehumidified air, such as the food in a refrigerator, loses moisture as the system 100 continually dehumidifies the air surrounding the food. The loss of moisture may cause freezer burn, result in a weight-loss, reduce nutrients, and cause adverse changes in appearance, such as color and texture, thus decreasing the marketability of the food with time. Furthermore, weight-loss results in the loss of value for foods sold by weight.
Accordingly, there is an ongoing need for heat transfer systems having an enhanced resistance to evaporator frosting during an on cycle of the compressor. The disclosed systems, methods, and devices overcome at least one of the disadvantages associated with conventional heat transfer systems.